1. Field of the Invention
The present invention relates generally to hydraulic systems and more particularly to a hydraulic warming system for use in low ambient temperature applications.
2. Prior Art
Operation of aircraft hydraulic systems requires stable system performance under a wide range of low ambient operating temperatures, typically 0xc2x0 to xe2x88x9265xc2x0 F. In aircraft systems requiring the use of hydraulic motors of the piston type (in-line or bent-axis), such as leading-edge slat actuation systems, trailing-edge flap actuation systems, horizontal stabilizer trim actuation systems and tilt rotor pylon conversion actuation systems, one of the critical issues facing the hydraulic system manufacturer is maintaining hydraulic motor performance at vehicle-specified levels under low ambient temperature motor operation. As the ambient temperature decreases, which is a normal operating environment for a hydraulic motor in an aircraft flying at high altitude (typically above 10,000 feet) or in low temperature operating zones such as Alaska or for a prolonged period of time, the viscosity of the hydraulic fluid flowing through the motor increases dramatically which, if not heeded, may severely downgrade the hydraulic motor performance and ultimately affect the entire aircraft with serious consequences.
Various solutions to this problem have been proposed in the prior art over the years with limited success. For example, U.S. Pat. No. 4,129,986 to Heinrich discloses a method and means for preventing thermal shock to a hydraulic motor in low-temperature environments, in which warm oil is circulated through the motor before use. The method also includes the step of returning the warm oil to the tank after it has passed through the motor and if appropriate, it may also include the preliminary step of lowering the warm oil pressure before it is passed through the hydraulic motor. This particular approach requires modification of the motor itself and adding several components to the basic hydraulic circuit which raises the overall system cost and thus may not be a suitable solution for most hydraulic system manufacturers.
Another example is U.S. Pat. No. 4,972,762 to Kubik which teaches the use of a warm-up circuit for hydraulic pilot control systems in which fluid heated by an idling pump is used to warm-up the hydraulic fluid in the pilot control system. The warm-up circuit includes a pair of branch conduits commonly connected to the high pressure side of the pilot pressure pump and respectively connected via flow restriction means to the pilots of a primary directional control valve. Fluid can flow from the branch conduits through the pilots and then through the centered pilot valve to the low pressure side of the pilot pump allowing a warm-up circulation of fluid through the entire pilot circuit while the control valve remains centered. This approach calls for a complicated and inefficient hydraulic motor warming setup which also adds to the overall hydraulic system cost.
Various other patents disclose variations on the same cost-prohibitive hydraulic motor warming approach. Therefore, the need arises for a novel cost-efficient method and system for continuously warming the internal rotating components of a hydraulic motor when the same is not operational. A hydraulic system of this type may employ a piston-type hydraulic motor driven by a two-stage jet pipe flow control electro-hydraulic servovalve having a built-in internal fluid flow restrictor which automatically heats the hydraulic fluid flowing through it. The novel cost-efficient method may involve taking the heated hydraulic fluid flow already provided continuously through the first stage internal leakage fluid flow path of the two-stage jet pipe flow control electro-hydraulic servovalve and continuously passing the heated hydraulic servovalve leakage fluid through the hydraulic motor to raise and maintain the temperature of the internal rotating components of the hydraulic motor well above the low ambient temperature of the environment. Use of this novel approach will greatly increase the marketability of the hydraulic system as a whole and provide a viable low cost solution for the hydraulic system manufacturer and user as well.
The present invention is directed to a hydraulic warming system for low ambient temperature operation comprising hydraulic servo valve means for providing a flow of pressurized hydraulic fluid from a source thereof to a plurality of hydraulic load driving passageways; means for controlling the flow of pressurized hydraulic fluid in the plurality of hydraulic load driving passageways; a hydraulic motor coupled between the flow controlling means and a hydraulic system return passageway and driven by the pressurized hydraulic fluid from the hydraulic servo valve means through the plurality of hydraulic load driving passageways, the hydraulic motor having internal rotating components; and means for continuously warming the internal rotating components of the hydraulic motor when the hydraulic motor is not operational to improve the performance and efficiency of the hydraulic motor in a low ambient temperature environment. The hydraulic servo valve means includes a two-stage jet pipe flow control electro-hydraulic servovalve. The servovalve includes an inlet port coupled to the source for receiving pressurized hydraulic fluid from the source, a plurality of outlet ports coupled respectively to the plurality of hydraulic load driving passageways, a fluid flow restrictor and a return port for releasing internal hydraulic leakage fluid at low flow rate from the electro-hydraulic servovalve through a return passageway, the low flow rate internal hydraulic leakage fluid being warmed by flowing through the fluid flow restrictor.
In accordance with one aspect of the present invention, the flow controlling means includes a spool and sleeve bypass-shutoff valve having a pre-loaded return spring, a plurality of inlet ports coupled respectively to the plurality of hydraulic load driving passageways and to the return passageway and a plurality of outlet ports. The spool of the bypass-shutoff valve is driven linearly by a solenoid valve coupled to the bypass-shutoff valve between a first position and a second position, the first position utilizing the warmed low flow rate hydraulic leakage fluid from the servovalve to provide continuous warming of the internal rotating components of the hydraulic motor when the hydraulic motor is not operational, the second position diverting the warmed low flow rate hydraulic leakage fluid from the servovalve directly to the hydraulic system return passageway bypassing the internal rotating components of the hydraulic motor when the hydraulic motor is operational, the servovalve being constantly pressurized through the inlet port from the source.
In accordance with another aspect of the present invention, the hydraulic motor further comprises a case drain cavity having a return port coupled to the hydraulic system return passageway for closing the hydraulic system circuit and a plurality of inlet ports coupled respectively to at least some of the plurality of outlet ports of the bypass-shutoff valve through a plurality of hydraulic motor passageways, the hydraulic motor driven by pressurized hydraulic fluid flowing through the plurality of hydraulic motor passageways, the driven hydraulic motor returning pressurized hydraulic fluid at high flow rate to the hydraulic system return passageway, the warmed low flow rate hydraulic leakage fluid from the servovalve being combined with the high flow rate return flow from the hydraulic motor when the spool of the bypass-shutoff valve is in the second position.
In accordance with yet another aspect of the present invention, the warming means includes the servovalve fluid flow restrictor, a warming fluid inlet port on the hydraulic motor and a hydraulic motor warming passageway coupled between one of the outlet ports of the bypass-shutoff valve and the warming fluid inlet port for continuously flowing the warmed low flow rate hydraulic leakage fluid from the servovalve to the internal rotating components of the hydraulic motor when the hydraulic motor is not operational and the spool of the bypass-shutoff valve is in the first position, the hydraulic motor returning the warmed low flow rate hydraulic leakage fluid to the hydraulic system return passageway through the case drain cavity return port.
In accordance with still another aspect of the present invention, the hydraulic warming system further comprises a hydraulic fluid distributor for integrating the hydraulic load driving passageways, the servovalve return passageway, the hydraulic motor passageways and the hydraulic motor warming passageway. The hydraulic fluid distributor is a valve manifold having an inlet port for coupling to the source, a servovalve supply passageway coupled between the servovalve inlet port and the valve manifold inlet port and an outlet port for coupling to the hydraulic system return passageway, the servovalve operatively coupled to the valve manifold. The valve manifold further includes a recess for integrating the hydraulic motor proximate to the operatively coupled servovalve, the case drain cavity of the hydraulic motor defined in the recess.
These and other aspects of the present invention will become apparent from a review of the accompanying drawings and the following detailed description of the preferred embodiments of the present invention.